Low pressure board or low pressure laminate is a well known, industrially important, and moderate to low cost product used in many industries including the furniture industry. In general, it is formed by a low pressure laminating procedure utilizing a suitable substrate and a decorative facing sheet or decor sheet which is impregnated with a thermosettable resin such as melamine-formaldehyde resin, often simply called "melamine resin", some other amino resin such as urea-formaldehyde resin, or an unsaturated polyester resin, and optionally with a similar resin impregnated barrier sheet interposed between the substrate and the decor sheet. The substrate can be formed of a variety of materials, such as thermosettable resin impregnated paper sheets, but more usually chipboard, fiberboard such as MASONITE.RTM. fiberboard, particle board or the like.
One of the problems which has occurred in the manufacture of low pressure board is the tendency of the backing board or substrate, particularly if it is a rough material such as chipboard or particle board, to be visible through the decorative facing paper. This phenomenon is known as "show-through" and, unless special efforts are taken, the texture and/or the color of the backing board may show through the decor sheet, which is commercially undesirable. Over the years, a number of expedients have been developed to deal with this problem, but these expedients invariably are unduly expensive, create additional problems and/or fail to fully solve the problem. In addition to the problem of show-through, the backing board, or substrate often has a tendency to drain the thermosettable impregnating resin from the decor sheet during pressing, i.e. the backing board seems to soak up the resin from the decor sheet during pressing, giving the product a poor appearance, e.g. a mottled appearance due to insufficient resin being present after lamination in the decorative facing sheet.
One solution to both the problem of show-through and migration of resin from the facing sheet to the substrate during pressing is the use of special facing paper having a very high density. However, this solution to the problem is expensive in two regards, i.e. the paper is expensive to begin with and, because of its high density, it is difficult to adequately impregnate with the thermosettable resin. Thus, while the use of high density facing paper is a partial solution to the aforementioned problems, it is available only at a substantially increased price which many customers are unwilling to pay.
Other proposed solutions to the above-mentioned problems have centered on back coating the decor paper with excess resin and the use of the optional barrier sheet. It has also been proposed to back coat the decor paper with a titanium dioxide filled acrylic resin in order to attempt to form a barrier layer between the decorative facing sheet and the substrate in order to reduce show-through as well as migration of the thermosettable resin from the facing sheet to the substrate. These techniques have not been fully satisfactory for the reasons noted above.
Microcrystalline cellulose, sold commercially under the trademark AVICEL, is an unusual colloidal form of cellulose which has found substantial use in a wide variety of industries, including the food, pharmaceutical and cosmetics industries as a suspending, extending, thickening and pseudoplastic agent. Microcrystalline cellulose has also been extensively used in the laminate industry as a binder material for abrasion resistant particles in order to impart abrasion resistance to the upper surfaces of such laminates, noting such patents as Scher et al U.S. Pat. No. 4,395,452 and its related patents; O'Dell et al U.S. Pat. No. 4,499,137; Ungar et al U.S. Pat. Nos. 4,713,138 and 5,037,694; and Lex et al U.S. Pat. No. 4,971,855, among others. Early patents dealing with microcrystalline cellulose are the Battista U.S. Pat. Nos. 3,157,518; 3,259,537 and 3,275,580; Battista is also the author of a book on microcrystalline cellulose. The Fuerst U.S. Pat. No. 3,373,071 shows a cast in situ overlay formed of microcrystalline cellulose, resin and grit. Scher et al U.S. Pat. No. 4,400,423 and its related patents show the use of a second layer of microcrystalline cellulose above a print sheet and below a top layer of microcrystalline cellulose and grit.
Insofar as is known, microcrystalline cellulose or the like has not been contemplated for use as a barrier layer coating between a decorative print sheet and a substrate in the manufacture of low pressure board.